A wireless communication system uses a layer configuration in which a protocol layer is divided into multiple layers and the divided layer is further divided into multiple sublayers. For example, in the 3rd Generation Partnership Project (3GPP), layer-2, which corresponds to a data link layer, includes three sublayers: a medium access control (MAC), a radio link control (RLC), and a packet data convergence protocol (PDCP).
FIG. 10 illustrates a protocol configuration of layer-2. As illustrated in FIG. 10, layer-2 has a MAC entity that belongs to a MAC sublayer, an RLC entity that belongs to an RLC sublayer, and a PDCP entity that belongs to a PDCP sublayer. Here, a transport channel is a service access point (SAP) that is defined between layer-1 (physical layer) and the MAC sublayer. A logical channel is a SAP that is defined between the MAC sublayer and the RLC sublayer. Furthermore, a radio bearer is a SAP that is defined by the PDCP layer that can multiplex multiple logical channels to the transport channel. Although not illustrated in FIG. 10, layer-1 is located at a lower layer in the MAC sublayer.
Communication devices arranged on the receiving end and the transmission end of a wireless communication system have a layer configuration as illustrated in FIG. 10. In the following, processes performed by a transmission-end entity and a receiving-end entity will be described.
From among sublayer entities in layer-2, a PDCP entity and an RLC entity are present in accordance with the number of logical channels (LCH) (n in FIG. 10) that are used for communication. The PDCP entity and the RLC entity performs packet data unit (PDU) transmission by associating with each other.
By using a bandwidth that can be used for data transmission or a radio resource, such as electrical power, a MAC entity on the transmission end determines free space for the MAC-PDU, appropriately allocates the RLC-PDU that is output from each of the n RLC entities to free space for the MAC-PDU, and performs multiplexing. Then, the MAC entity adds a MAC header to the multiplexed RLC-PDU (packet data unit) and transfers the obtained MAC-PDU to a hybrid automatic repeat request (HARQ).
In contrast, a MAC entity on the receiving end analyzes the PDU transferred from the HARQ, divides the PDU into two or multiple RLC-PDUs, and transfers them to each RLC entity. Then, the RLC entity on the receiving end analyzes the RLC-PDU, constructs an RLC-SDU, and transfers it to the PDCP entity.
Furthermore, in a transmission process, by holding the MAC-PDU at the time of transmission, the HARQ on the transmission end performs an error correction process and a cyclic redundancy check (CRC) encoding on the MAC-PDU.
In contrast, when received information indicates a reception error, i.e., when an error detection result of the CRC coding indicates a negative, an HARQ on the receiving end replies to the other end (transmission end) with a negative acknowledgment (NACK). When received information is acceptable, i.e., when an error detection result of the CRC code indicates a positive, the receiving end replies to the other end (transmission end) with an acknowledgement (ACK). Furthermore, when the MAC entity on the transmission end receives a NACK, the MAC entity re-transmits the subject MAC-PDU. When the MAC entity receives an ACK, the MAC entity cancels the MAC-PDU that is held at the time of initial transmission and transmits a new MAC-PDU.
Furthermore, when the MAC entity does not receive an ACK even when the MAC entity repeatedly re-transmits a single MAC-PDU by the number of times corresponding to the maximum number of re-transmissions, the MAC entity cancels the subject MAC-PDU. In such a case, the RLC entity performs retransmission control using an automatic repeat request (ARQ) that uses poll/status information.
In recent years, the LTE-advanced edition, which is a function-extended edition of Long Term Evolution (LTE), is being studied as part of the 3GPP and therefore there are plans to introduce relay stations (RS). FIG. 11 illustrates a system configuration when an RS is used. In (1) of FIG. 11, a system configuration when an RS is not used is illustrated. In (2) of FIG. 11, a system configuration when an RS is used is illustrated. In FIG. 11, an evolved Node B (eNB) indicates a base station and the symbol UE indicates user equipment.
As illustrated in FIG. 11, when an RS is not used, downlink or uplink communication is performed between the eNB and the UE. Then, when an RS is used in the future, the downlink or the uplink communication is performed among the eNB, the RS, and the UE.
When an RS is used, the eNB and the UE usually have three sublayers in layer-2 as the protocol stack; however, the number of sublayers included in layer-1 or layer-2 in an RS has not been determined.
Non-Patent Document 1: “Evolved Universal Terrestrial Radio Access (E-UTRA); LTE-Advanced (Release 8)” 3GPP TR 36.913 V8.0.1 (2009-03)
However, the features of sublayers included in layer-1 and layer-2 in an RS have advantages and disadvantages; therefore, an optimum protocol is not configured even when an RS includes a sublayer of layer-1 or layer-2 functioning as a termination. Specifically, when a termination of a layer included in the RS is fixed, there may be a case in which the feature of the layer is not suitable for a radio environment, thereby an unsuitable protocol is configured.
In the following, for example, it will be assumed that the description is of a downlink of eNB-RS-UE when the RS has layer-1. In such a case, because a transport block (TrBLK) passes the RS without the size of the TrBLK being adjusted in the RS, only the data having the same size is transmitted between the eNB-RS and between the RS-UE. Accordingly, the data size of the TrBLK between one section (between RS-UE) is determined on the basis of the radio environment of the other section (between eNB-RS). In other words, the data size of the TrBLK is not changed between the RS-UE.
Accordingly, when a radio environment of the eNB-RS is different from that of the RS-UE, it is not possible to configure an optimum protocol in an RS. For example, when a radio environment of the RS-UE is worse than that of the eNB-RS, the error rate of the data becomes high, thus the data is less likely to be delivered. In contrast, when a radio environment of RS-UE is better than that of the eNB-RS, a TrBLK having data size larger than that of the eNB-RS can be transmitted. However, it is only possible to transmit the TrBLK having the same data size as that of RS-UE.